ABSTRACT: OVERALL Red blood cell transfusion remains a life-saving therapy for patients with sickle cell disease (SCD). A major problem is the high rate of alloimmunization (antibody formation against transfused red cells) that occurs in transfused patients with SCD. Genetic diversity in blood group antigens in patients of African descent compared to the primarily European-based donor pool contributes to this high incidence and complexity of antibodies found in patients with SCD. Finding compatible red blood cell (RBC) units is often complicated by a lack of rare reagent RBCs to properly identify these complex antibody specificities. This complication delays care, increases costs, and makes transfusion therapy impossible for some patients. The ultimate goal of this proposal is to use human induced pluripotent stem cells (iPSCs) to produce standard and reliable red blood cell (RBC) reagents to resolve this major problem. We have designed a panel of iPSCs genetically engineered to express unique combinations of blood group antigens (customized iPSCs) that are difficult or virtually impossible to find in donor RBCs. RBC reagents produced from these customized iPSCs will provide the means to streamline and standardize antibody identification in alloimmunized patients, and ultimately can be used as ?universal? donor cells for future therapeutic applications. Our efforts will address several existing challenges that include: i. insufficient or no living blood donors expressing the combinations of blood group antigens needed to resolve the complex antibody specificities in patients with SCD, ii. lack of iPSC differentiation protocols to produce definitive, adult-type RBCs without the use of serum or stromal cells, and iii. the prohibitively expensive manufacturing costs of iPSC-derived RBCs. In this U01 application, we propose three integrated Projects from a group of highly collaborative investigators with expertise in areas that can address all three challenges and will drive the field forward by providing innovative solutions to these current obstacles.